AIDS, Acquired Immune Deficiency Syndrome, is a catastrophic disease that has reached global proportions. AIDS was first brought to the attention of the Center for Disease Control and Prevention (CDC) in 1981 when seemingly healthy homosexual men came down with Karposi's Sarcoma (KS) and Pneumocystis carinii pneumonia (PCP), two opportunistic diseases that were only known to afflict immunodeficient patients. A couple of years later, the causative agent of AIDS, a lymphoadenopathy-associated retrovirus, the human immunodeficiency virus (HIV) was isolated by the Pasteur Institute in Paris, and later confirmed by an independent source in the National Cancer Institute in the United States.
Another virus that causes a serious human health problem is the hepatitis B virus (HBV). HBV is second only to tobacco as a cause of human cancer. The mechanism by which HBV induces cancer is unknown. It is postulated that it may directly trigger tumor development, or indirectly trigger tumor development through chronic inflammation, cirrhosis, and cell regeneration associated with the infection.
After a 2- to 6-month incubation period in which the host is unaware of the infection, HBV infection can lead to acute hepatitis and liver damage, resulting in abdominal pain, jaundice and elevated blood levels of certain enzymes. HBV can cause fulminant hepatitis, a rapidly progressing, often fatal form of the disease in which large sections of the liver are destroyed.
Patients typically recover from the acute phase of HBV infection. In some patients, however, high levels of viral antigen persist in the blood for an extended, or indefinite, period, causing a chronic infection. Chronic infections can lead to chronic persistent hepatitis. Patients infected with chronic persistent HBV are most common in developing countries. By mid-1991, there were approximately 225 million chronic carriers of HBV in Asia alone, and worldwide, almost 300 million carriers. Chronic persistent hepatitis can cause fatigue, cirrhosis of the liver, and hepatocellular carcinoma, a primary liver cancer.
In western industrialized countries, the high-risk group for HBV infection includes those in contact with HBV carriers or their blood samples. The epidemiology of HBV is very similar to that of AIDS, which is a reason why HBV infection is common among patients infected with HIV or AIDS. However, HBV is more contagious than HIV.
In 1985, it was reported that the synthetic nucleoside 3′-azido-3′-deoxythymidine AZT, Zidovudine, Retrovir) inhibits the replication of HIV and became the first FDA-approved drug to be used in the fight against AIDS. Since then, a number of other synthetic nucleosides, including 2′,3′-dideoxyinosine (DDI), 2′,3′-dideoxycytidine (DDC), 2′,3′-dideoxy-2′,3′-didehydrothymidine (D4T), (−)-2′,3′-dideoxy-3′-thiacytidine (3TC), and (−)-carbocyclic 2′,3′-didehydro-2′,3′-dideoxyguanosine (carbovir) and its prodrug abacavir, have been proven to be effective against HIV. After cellular phosphorylation to the 5′-triphosphate by cellular kinase, these synthetic nucleosides are incorporated into a growing strand of viral DNA, causing chain termination due to the absence of the 3′-hydroxyl group. They can also inhibit the viral enzyme reverse transcriptase.
Both and 3TC and its 5-fluorocytosine analog (FTC) exhibit activity against HBV. Furman, et al., “The Anti-Hepatitis B Virus Activities, Cytotoxicities, and Anabolic Profiles of the (−) and (+) Enantiomers of cis-5-Fluoro-1-[2-(Hydroxymethyl)-1,3-oxathiolane-5-yl]-Cytosine” Antimicrobial Agents and Chemotherapy, December 1992, pp. 2686-2692; and Cheng, et al., Journal of Biological Chemistry, Volume 267(20), pp.13938-13942 (1992).
The discovery that a racemic oxathiolane nucleoside BCH-189 possessed a potent activity against replication of human immunodeficiency virus (HIV) (Belleau, B. et al., 5th International Conference on AIDS, Montreal, Canada, Jun. 4-9, 1989, #T.C.O. 1) prompted Chu et al. to synthesize the chiral products (+)- and (−)-BCH-189 (Tetrahedron Lett., 1991, 32, 3791). The latter, lamivudine, otherwise known as 3TC or epivir, is currently used clinically in the treatment of both HIV infection and HBV infection. 3TC and interferon are currently the only FDA-approved drugs for the treatment of HBV infection. Viral resistance typically develops within 6 months of 3TC treatment in about 14% of patients.
It was later determined that the 5-fluorocytosine analogue, (−)-FTC, is even more active against HIV (Choi, W. et al., J. Am. Chem. Soc., 1991, 113, 9377). More recently, the racemic form of FTC or Racivir has been discovered to show more beneficial effects against HIV or HBV than (−)-FTC alone (Schinazi, R. F., et al., Antimicrobial Agents Chemotherapy 1992, 2423, U.S. Pat. Nos. 5,204,4665; 5,210,085; 5,914,331; 639,814). Cis-2-hydroxymethyl-5-(5-fluorocytosin-1-yl)-1,3-oxathiolane (FTC) is currently in clinical trials for the treatment of HIV and separately for HBV. See Schinazi, et al., (1992) “Selective inhibition of human immunodeficiency viruses by racemates and enantiomers of cis-5-fluoro-1-[2-(hydroxymethyl)-1,3-oxathiolane-5-yl]cytosine” Antimicrob. Agents Chemother. 2423-2431; U.S. Pat. No. 5,210,085; WO 91/11186; WO 92/14743; U.S. Pat. No. 5,914,331 and U.S. Pat. No. 5,814,639.
These 1,3-oxathiolane nucleosides are manufactured by condensation of silylated purine or pyrimidine base with a 1,3-oxathiolane intermediate. U.S. Pat. No. 5,204,466 discloses a method to condense a 1,3-oxathiolane with a silylated pyrimidine using tin chloride as a Lewis acid, which provides virtually complete β-stereoselectivity (see also Choi et al., loc. cit.). A number of U.S. patents disclose processes for the preparation of 1,3-oxathiolane nucleosides via condensation of a 1,3-oxathiolane-2carboxylic acid ester with a protected silylated base in the presence of a silicon-based Lewis acid, followed by reduction of the ester to the corresponding hydroxymethyl group to afford the final product (see U.S. Pat. Nos. 5,663,320; 5,693,787; 5,696,254; 5,744,596; 5,756,706 and 5,864,164). In addition, these patents contain generic disclosures for the synthesis of 1,3-dioxolane nucleosides in a similar fashion using the corresponding 1,3-dioxolane intermediate.
U.S. Pat. No. 5,272,151 discloses a process using a 2-O-protected-5-O-acylated-1,3-oxathiolane for the preparation of nucleosides by condensation with a silylated purine or pyrimidine base in the presence of a titanium catalyst.
U.S. Pat. No. 6,215,004 discloses a process for producing 1,3-oxathiolane nucleosides that includes condensing 2-O-protected-methyl-5-chloro-1,3-oxathiolane with a silylated 5-fluorocytosine without a Lewis acid catalyst.
In all cases, the 1,3-oxathiolane ring is prepared in one of the following ways: (i) reaction of an aldehyde derived from a glyoxylate or glycolic acid with mercaptoacetic acid in toluene in the presence of p-toluenesulfonic acid to give 5-oxo-1,3-oxathiolane-2-carboxylic acid (Kraus, J-L., et al., Synthesis, 1991, 1046); (ii) cyclization of anhydrous glyoxylates with 2-mercaptoacetaldehyde diethylacetal at reflux in toluene to give 5-ethoxy-1,3-oxathiolane lactone (U.S. Pat. No. 5,047,407); (iii) condensation of glyoxylic acid ester with mercaptoacetaldehyde (dimeric form) to give 5-hydroxy-1,3-oxathiolane-2-carboxylic ester or (iv) coupling of an acyloxyacetaldehyde with 2,5-dihydroxy-1,4-dithiane, the dimeric form of 2-mercaptoacetaldehyde to form a 2-(ayloxy)methyl-5-hydroxy-1,3-oxathiolane. The lactone, 5-oxo compound, has to be reduced to the corresponding lactol during the process to synthesize nucleosides. The 2-carboxylic acid or its ester also has to be reduced to the corresponding 2-hydroxymethyl derivatives with borane-methylsulfide complex.
The key intermediate, aldehyde, can be prepared using several methods: (i) lead tetraacetate oxidation of 1,4-di-O-benzoyl meso-erythritol (Ohle, M., Ber., 1941, 74, 291), 1,6-di-O-benzoyl D-mannitol (Hudson, C. S., et al., J. Am. Chem. Soc., 1939, 61, 2432) or 1,5-di-O-benzoyl-D-arabitol (Haskins, W. T., et. al., J. Am. Chem. Soc., 1943, 65, 1663); (ii) preparation of monoacylated ethylene glycol followed by oxidation to aldehyde (Sheikh, E., Tetrahedron Lett., 1972, 257; Mancuso, A. J. and Swern, D., Synthesis, 1981, 165; Bauer, M., J. Org. Chem., 1975, 40, 1990; Hanessian, S., et al., Synthesis, 1981, 394); (iii) acylation of ethylene chlorohydrin followed by dimethylsulfoxide oxidation (Kornblum, N., et al., J. Am. Chem. Soc., 1959, 81, 4113); (iv) acylation of 1,2-isopropylideneglycerol followed by deacetonation and periodate oxidation (Shao, M-J., et al., Synthetic Commun., 1988, 18, 359; Hashiguchi, S., et al, Heterocycles, 1986, 24, 2273); (v) lead tetraacetate oxidation (Wolf, F. J., Weijlard, J., Org. Synth., Coll. Vol., 1963, 4, 124); (vi) ozonolysis of allyl or 3-methyl-2-buten-1-ol acylate (Chou, T.-S., et al., J. Chin. Chem. Soc., 1997, 44, 299; Hambeck, R., Just, G.; Tetrahedron Lett., 1990, 31, 5445); (vii) and more recently, by acylation of 2-butene-1,4-diol followed by ozonolysis (Marshall, J. A., et al., J. Org Chem., 1998, 63, 5962). Also, U.S. Pat. No. 6,215,004 discloses a process to prepare acyloxyacetaldehyde diethylacetal by acylation of 2,2-diethoxyethanol.
α-Acyloxyacetaldehyde is the key intermediate not only for the synthesis of those oxathiolane and dioxolane nucleosides but also for the synthesis of biologically active compounds, such as mescarine (Hopkins, M. H., et al., J. Am. Chem. Soc., 1991, 113, 5354), oxetanocin (Hambalek, R., Just, J., Tetrahedron Lett., 1990, 31, 5445), kallolide A (Marshall, J. A., et al J. Org. Chem., 1998, 63, 5962), (±)-kumausallene and (+)-epi-Kumausallene (Grese, T. S., et al., J. Org. Chem., 1993, 58, 2468), 1,3-dioxolane nucleosides.
Norbeck, D. W. et al. (Tetrahedron Letters 1989, 30, 6263) reported a synthesis of (±)-1-[(2β,4β)-2-(hydroxymethyl)-4-dioxolanyl]thymine (“(±)-dioxolane-T”) that results in a racemic mixture of diastereomers at the C4′ atom. The product is a derivative of 3′-deoxythymidine in which the C3′ atom has been replaced by an O3′ atom. The product was synthesized in five steps from benzyloxyaldehyde dimethylacetal and (±)-methyl glycerate, resulting in a 79% yield of the 1:1 diastereomeric mixture. Norbeck reported that the racemic mixture of (±)-dioxolane-T showed moderate anti-HIV activity in ATH8 cells (EC50 of 20 μM; Tetrahedron Letters 1989, 30, 6246).
Belleau et al. (5th International Conference on AIDS; Montreal, Canada; Jun. 4-9, 1990; Paper No. TCO1) reported a method of synthesizing cytidine nucleosides that contain oxygen or sulfur in the 3′-position. The dioxolane ring was prepared by condensation of RCO2CH2CHO with glycerin. As with Norbeck's procedure, the Belleau synthesis results in a racemic mixture of diastereomers about the C4′ carbon of the nucleoside. Belleau reported that the sulfur analog, referred to as (±)-BCH-189, had anti-HIV activity. (±)-Dioxolane-T was also synthesized in similar fashion by Choi et al. (Choi, W.-B et al., J. Am. Chem. Soc. 1991, 113, 9377-9378; Liotta, D. C. et al. U.S. Pat. No. 5,852,027).
An asymmetric synthesis of dioxolane nucleosides was reported by Evans, C. A. et al. (Tetrahedron: Asymmetry 1993, 4, 2319-2322). Reaction of D-mannitol with BnOCH2CH—(OCH3)2 in the presence of SnCl2 in 1,2-dimethoxyethane followed by RuCl3/NaOCl oxidation gave cis- and trans-dioxolane-4-carboxylic acid, which was then converted to D- and L-dioxolane nucleosides by decarboxylation, coupling and deprotection reactions. An alternative and more efficient route to these carboxylic acids by reaction of BnOCH2CH(OCH3)2 with L-ascorbic acid was also reported in the paper.
The chiral carboxylic acid (6) can also be prepared by reacting commercially available 2,2-dimethyl-1,3-dioxolane-4-(S)-carboxylic acid with a protected derivative of hydroxy-acetaldehyde such as benzoyloxyacetaldehyde, under acidic conditions (Mansour, T. et al, U.S. Pat. No. 5,922,867; Nghe Nguyen-Ba, U.S. Pat. No. 6,358,963).
The antiviral activity of dioxolane nucleosides (see Corbett, A. H. & Rublein J. C., Curr. Opin. Investig. Drugs 2001, 2, 348-353; Gu, Z., et al., Antimicrob. Agents Chemother. 1999, 43, 2376-2382; Gu, Z., et al., Nucleosides Nucleotides 1999, 18, 891-892) prompted Chu et al. to synthesize a series of analogs enantiomerically in a search for potent antiviral and/or anticancer agents. Among them, several compounds such as (−)-(2′R,4′R)-2,6-diamino-9-[2-(hydroxy-methyl)-1′,3′-dioxolan-4′yl]purine (DAPD) (U.S. Pat. No. 5,767,122), (−)-(2S,4R)-1-[2-(hydroxymethyl)-1,3-dioxolan-4yl]cytosine) L-OddC) and (−)-(2′S,4′R)-1′-[2′-(hydroxy-methyl)-1′,3′-dioxolan-4′-yl]-5-iodouracil) L-IOddU (U.S. Pat. No. 5,792,773) have been identified and are currently in pre-clinical or clinical studies to assess their value as antiviral or anticancer agents (see Kim, H.-O. et al., J. Med. Chem. 1993, 36, 519-528 and references therein; Corbett, A. H. & Rublein J. C., Curr. Opin. Investig. Drugs 2001, 2, 348-353; Gu, Z., et al., Antimicrob. Agents Chemother. 1999, 43, 2376-2382; Mewshaw, J. P., et al., J. Acquir. Immune Defic. Syndr. 2002, 29, 11-20).
As one enantiomer of racemic nucleosides usually shows higher biological activity (see Kim et al., J. Med. Chem. 1993, 36, 519-528 and references therein), Chu et al. developed methods for the asymmetric synthesis of dioxolane nucleosides from D-mannose and L-gulonic lactone for D- and L-dioxolane nucleosides, respectively (U.S. Pat. Nos. 5,767,122, 5,792,773). However, these processes involved many steps and most of the intermediates needed to be purified by silica gel column chromatography (see Kim et al. J. Med. Chem. 1993, 36, 519-528 and references therein).
To prepare a sufficiently large quantity of dioxolane nucleoside drug substance for clinic trials, a chiral 2-acyloxymethyl-5-oxo-1,3-dioxolane has been used as the key intermediate. It was prepared by cyclization of ROCH2CHO or its acetal with glycolic acid in the presence of BF3, followed by column separation on chiral resin or by enzymatic resolution. It is very expensive to prepare the chiral lactone by these processes, because of the high cost of the chiral resin and enzymes.
Thus, there remains a need for cost-effective and stereoselective processes to produce biologically active isomers of dioxolane nucleosides.
It is an object of the present invention to provide novel and cost-effective processes for the synthesis of enantiomerically pure dioxolane nucleosides.